Blender Surface Roughness
Solution is simple:


You go in edit mode and select desired face.
Click W, than select Subdivide.
Either repeat previous step multiple times or on the bottom left part of screen you will see a value Number of Cuts that you can adjust to your needs.
Select the subdividen faces and in Mesh tab add a new vertex grop and assign those objects to it.
Add displacement modifier, select the verex group under it's setting, select texture and adjust values.


Done! ;)